One technique, called strand-to-strand brazing is a procedure for brazing series loops of a strand-to-strand configuration. After stator bar assembly and during stator wind, the top and bottom strands are cut to size, bent into position and brazed layer by layer. Examples of this technique are illustrated in FIGS. 1-5.
One technique, called strand-to-strand brazing is a procedure for brazing series loops of a strand-to-stand configuration. After stator bar assembly and during stator wind, the top and bottom strands are cut to size, bent into position and brazed layer by layer. Examples of this technique are illustrated in FIGS. 1-5.
Referring to FIGS. 1-3, two inner strands 10 are bent down from the top bar into position and cutoff with special cutters. Two inner strands 12 from the bottom bar are then bent into position to overlap the top strands 10 and cut. The minimum overlap of brazed strands is about three times the thickness of the strand, e.g., at least about 0.37 inches. The overlap is always made equal to or greater than the width of the copper being brazed. The brazes are made over an arc of approximately 90 degrees (FIG. 1) although this angle may be limited to accessible space for braze equipment. The gap between consecutive sets of brazes is nominal, e.g., 0.090 inches to 0.160 inches. When brazing a joint, it is separated from previous joints by a mica strip 14. As a consequence, radial separation between adjacent joints is, e.g., a maximum of 0.03 inches including the mica separator thickness. Before brazing, a braze alloy ribbon 16 is centered in the overlap area to be brazed.
For joints of one strand over one strand, the braze alloy ribbon is inserted as illustrated in FIGS. 2 and 3. Brazes may be 1, 2, or 4 strands wide. For joints of 2 strands over 2 strands, as illustrated in FIGS. 4 and 5, braze alloy 16 ribbon is inserted to cover all areas to be brazed. The eight strands are brazed in one operation, two wide by four deep. A brazing fixture or clamps are placed such that the overlapping strands are held tightly and in alignment, and exerting enough pressure to cause the pre-placed braze alloy 16 to flow and fill the joint as it melts when heat is applied. Heat is applied such that the joint is heated rapidly, but evenly to a brazing temperature of 1292° F. to 1364° F. (700° C. to 740° C.). The assembly reaches the proper temperature when the strands are dull cherry red in color and the pre-placed braze alloy flows. Face fed braze alloy may be added as needed to ensure that the joint is effectively filled, but not so much as to form drips. The assembly is then allowed to cool and solidify, after which pressure is released and it is visually inspected.
As is apparent, strand-to-strand brazing provides great flexibility because the bars can be cut to size as needed during winding, but assembly is time consuming and thus costly.
Another braze technique for series connections is referred to as the single shot braze, which is used for making series loops in a generator wound with solidified stranded stator bars. With this technique, a first braze process is carried out during stator bar manufacturing to solidify the stranded bar package ends. A second braze process is then performed after stator bar assembly (during stator wind) to complete top to bottom bar series loop connections. Since the method uses only one braze per series loop in assembly, it has been coined the one or single shot braze method. For brazing solidified strands to solid phase connections (not shown), a solid connection strap is used to sandwich the strands, using a copper plate on one side of the solidified strands and the connection strap on the other. Bar to bar (series) loops, as schematically illustrated in FIG. 6 have a copper plate 36 on each side of the solidified strands 32, 34 to form a sandwich and are brazed as one unit. Thus, two plates 36 are assembled between a top and bottom bar 32, 34 as illustrated, and clamps (not shown) are used to hold the plates in place. The braze is performed by heating the area around the top, radially inner, bar 32 first until braze temperature is reached (dull red color—1350° F.) and adding braze alloy around all four sides of the joint for each plate. Then the installer switches to the other ends of the plates at the bottom bar 34 (the stator bar seated in the bottom of the slot) and completes that part of the braze in the same manner.
As illustrated in FIG. 7, for brazing non-solidified stator bar 20 to solid phase strap connection, the strand package 20 is received between the phase connection strap 22 and backer plate 24. Braze alloy (sheets and stick) 26 between the stranded tiers, between the stranded tiers and the strap and plate, respectively, and between the strap 22 and plate 24 solidifies the assembly axially and radially over the entire joint.